The present invention relates generally to automatic control systems of the on/off mode type and more specifically to improvements in on/off mode control systems for better regulation about the system set point.
A fixed-bed coal gasifier is used to produce low BTU gas from the partial combustion of granular coal. Coal is introduced into the reaction chamber above the reaction zone and must be maintained at a prescribed level above the zone to maintain the desired product gas quality.
A coal feeder of the pocket type is used to feed coal into a fixed-bed coal gasifier. The coal feeder is of the conventional rotating pocket type. The speed control of the pocket rotors of the feeder is by means of a variable sheave drive. The sheave effective diameter is changed to vary the speed ratio from a constant speed electric motor prime mover. The sheave diameter is varied by means of a positioner that is electric motor driven in an on/off control mode. By running the positioner motor in forward or reverse directions, the feeder speed is increased or decreased. When the positioner motor is stopped, the feeder speed is maintained. One problem associated with the variable speed drive for automatic control is that it can not withstand the frequent speed jogging that it would be subjected to when used in a conventional automatic control system. Frequent jogging would quickly wear out the drive system and electrical control contacts.
Another problem is that the characteristics of this type of system which uses on/off mode control is that it will not allow large integral action to be used in controlling about a set point due to the lag introduced by the inherent system deadband. In the described system, the 90.degree. phase lag of the feeder/bed integrating response function causes control loop cycling (oscillation) when even a small amount of integral action is used.
Further, the disclosed system introduces an additional control problem due to the actuation of the gasifier stirrer approximately every 15 minutes, which corresponds to the stirrer vertical travel cycle time. This produces about a 20 percent variation in the nuclear level detector's output caused by shadowing from the stirrer and the shaft coupling the drive to the stirrer. Because of this large unavoidable cyclical variation, only a limited proportional action (gain&lt;1) can be used without undue jogging of the feeder speed drive. A gain less than one without accompanying integral action causes a considerable offset (steady state difference between set point bed level and achieved bed level) that could normally be eliminated by adding integral action. How fast the offset is removed when integral action is used depends upon the integral rate in repeats/minute set in the controller. Also, the controller must have adequate combined proportional/integral action to react to load perturbations and upsets such that large changes will not occur in the controlled bed level. A combination of gain=0.5 and an integral rate of 0.05 repeats/minute is barely adequate but still cycles as will be shown hereinbelow. If integral action is reduced to eliminate cycling, then too little control action results or if proportional action is increased, excessive jogging of the feeder speed control occurs.
Thus, there is a need for improvements in control systems for on/off mode controllers to provide improved regulation and prevent cycling of the control loop.